Photodiodes are frequently used as devices for converting optical signals to electrical signals at high speed. Photodiodes are indispensable in the fields of information processing and communication.
Various types of photodiodes are known, but a representative example of a photodiode for operating at high speed is the pin-type photodiode. As shown in FIG. 1, a pin-type photodiode is constituted from a semiconductor such as silicon and is of a configuration in which i-layer (intrinsic semiconductor layer) 51 is interposed between p-layer (p-type semiconductor layer) 52 and n-layer (n-type semiconductor layer) 53. P-layer 52 is formed as a thin layer on a portion of the surface of i-layer 51, and first electrode (anode electrode) 54 with window 59 formed in its center is provided so as to both enclose the periphery of p-layer 52 and contact p-layer 52. Second electrode (cathode electrode) 55 is provided on the surface of n-layer 53 that is not the i-layer 51 side. P-layer 52 is exposed on the bottom surface of window 59, and antireflection film 58 is provided on the exposed surface of p-layer 52.
Load resistance 50 and this pin-type photodiode are connected in a series to bias power supply 56, and when a reverse bias voltage is applied to the photodiode by bias power supply 56 such that the first-electrode 54 side is negative and the second-electrode 55 side is positive, substantially the entire region of i-layer 51 of high resistance becomes a depletion layer of the electric charge carrier. When incident light 57 irradiates the interior of the photodiode by way of window 59 in this state, the photons of incident light 57 are mainly absorbed by i-layer 51 to generate electron-hole pairs. The generated electrons and holes each drift in opposite directions within the depletion layer under the influence of the reverse bias voltage to generate a current, and are detected as a signal voltage between both ends of load resistance 50.
The chief factors that limit the response speed of the photoelectric conversion of such a pin-type photodiode are a circuit time constant that is determined by the product of load resistance 50 and the electrical capacitance produced by the depletion layer and the carrier transit time needed for the electrons and holes to pass through the depletion layer. Thus, in order to improve the response time, either the circuit time constant should be reduced or the carrier transit time should be shortened.
Schottky photodiodes are sometimes used to improve the response speed by shortening the carrier transit time. As shown in FIG. 2, a Schottky photodiode is composed of a semiconductor such as silicon, n−-type semiconductor layer 61 being formed on n+-type semiconductor layer 60, and further, semi-transparent metal film 66 being provided on a portion of the surface of n−-type semiconductor layer 61 so as to contact n−-type semiconductor layer 61. Semi-transparent metal film 66 is a metal thin-film thin enough to transmit incident light 67. First electrode 62 in which window 69 is formed in the center is provided so as to both surround the periphery of semi-transparent metal film 66 and contact semi-transparent metal film 66. Second electrode 63 is provided on the surface of n+-type semiconductor layer 60 that is not the n−-type-semiconductor layer 61 side. Semi-transparent metal film 66 is exposed on the bottom of window 69, and antireflection film 68 is provided on the exposed surface of semi-transparent metal film 66. As in the case of the pin-type photodiode shown in FIG. 1, a reverse bias voltage is applied by way of bias power supply 64 and load resistance 65 to first electrode 62 and second electrode 63.
In this type of Schottky photodiode, a Schottky barrier is generated in the vicinity of the interface in which n−-type semiconductor layer 61 contacts semi-transparent metal film 66. In the vicinity of this Schottky barrier, electrons are diffused from semi-transparent metal film 66 and toward n−-type semiconductor layer 61 to generate a depletion layer. When incident light 67 is irradiated in this state, electrons are generated in n−-type semiconductor layer 61, and these electrons drift within the depletion layer under the influence of the reverse bias voltage. The drift of the electrons within the depletion layer generates a current and is detected as a signal voltage between the two ends of the load resistance 65.
In addition, the light absorption in the device surface layer can be effectively used in a Schottky photodiode.
On the other hand, when the circuit time constant is reduced to improve the response speed of photoelectric conversion, either the load resistance should be reduced or the electrical capacitance of the depletion layer should be reduced. However, when the load resistance is reduced to shorten the circuit time constant, the voltage level of the regeneration signal that can be extracted drops, the device becomes more susceptible to the effect of thermal noise and other noises, and the SN ratio (signal-to-noise ratio) is degraded. Thus, reducing the electrical capacitance of the depletion layer results in the necessity to improve the SN ratio of the regeneration signal to reduce read errors. In particular, when the depletion layer is thinned to shorten the carrier transit time, the electrical capacitance increases, and as a result, the area of the depletion layer or the Schottky junction must be reduced to obtain higher speeds. However, decreasing the junction area reduces the utilization of the signal light, and this in turn gives rise to the problem of degradation of the SN ratio of the regeneration signal. Although a lens can be used to condense and improve the utilization of the signal light, the provision of a lens not only increases the size of the photoelectric conversion device itself, but also entails the difficult operations of aligning the lens and photodiode and positioning the lens and optical fiber.
In response to these problems, various attempts have been made with the development of technology in recent years to achieve a photoelectric conversion device of this type that is capable of higher speeds and more compact sizes than the prior art through the use of a metal surface plasmon
JP-A-59-108376 (Patent Document 1) discloses a photodetector that is composed of a metal/semiconductor/metal (MSM) device in which two electrodes are disposed on the same surface of a semiconductor. This MSM photodetector is generally a type of Schottky photodiode having a Schottky barrier in the vicinity of the two electrodes. A portion of the light transmitted by the electrodes is absorbed; by the semiconductor to generate free electrons. This type of MSM photodetector suffers from the problem that increasing the thickness of the semiconductor to raise the quantum efficiency causes an increase in the propagation distance of electrons and a consequent drop in the operation speed. To prevent this drop in operation speed, JP-A-59-108376 discloses an arrangement in which metal electrodes are provided along periodic surface irregularities to achieve efficient coupling of incident light and the surface plasmon of the metal electrodes and propagate the light within the photodetector. As one method of application to the fabrication of the above-described MSM photoreception device, JP-A-08-204225 (Patent Document 2) discloses a method for forming a metal film on a semiconductor and then oxidizing a portion of the metal film to form a phototransmissive insulation pattern.
A photoreception device that detects near-field light has also been proposed. JP-A-08-204226 (Patent Document 3) discloses an MSM photoreception device having a pair of conductive voltage application members on the same surface of a semiconductor in which the width of a phototransmissive insulation pattern separating the pair of conductive voltage application members is set to a dimension equal to or less than the wavelength and in which the near-field light generated from the ends of conductive voltage application members on the both sides of the phototransmissive insulation pattern is used to raise the response speed of photodetection. The conductive voltage application members are generally constituted by a metal film. In this configuration, the width of the opening for generating near-field light determines the efficiency and the distance of drift of electrons in the depletion layer determines the response speed, but because the width of the phototransmissive insulation pattern is the width of the depletion layer as a Schottky photodiode, the width of the opening and the distance of drift of electrons cannot be independently set and high efficiency and high speed therefore cannot be simultaneously obtained in the photoreception device.
JP-A-10-509806 (Patent Document 4) discloses a photoelectric coupler in which the surface plasmon phenomenon is used. In this photoelectric coupler, a device configuration is employed in which interdigital metal electrodes aligned with regular spacing on a semiconductor are arranged such that positive electrodes and negative electrodes confront each other with one fitting into the other. By means of this device configuration, incident light, transmitted light, reflected Flight, surface plasmon and the like are coupled with each other by resonance. In an MSM photoreception device of this type that uses photoelectric coupling technology, free electrons generated by incident light are enhanced by the coupling of incident light and surface plasmon, but when the area irradiated by incident light is reduced to reduce the electric capacitance of the depletion layer, the strength of the detected signal falls and the SN ratio drops.
JP-A-2002-076410 (Patent Document 5) discloses a photovoltaic device for converting the energy of sunlight to electrical energy in which a plurality of micro-semiconductors having a spherical or hemispherical shape and having pn bonding are used, each semiconductor sphere being interposed between a pair of electrodes and periodically arranged openings or depressions being provided on one of the pair of electrodes. The periodic shape provided on one of the electrodes causes the incident light and the surface plasmon to resonate, thus improving the photoelectric conversion efficiency of photovoltaic device as a whole. However, this technique relates to a photovoltaic device, i.e., a solar battery, in which high speed is not required in the response speed of photoelectric conversion. As a result, no investigation has been conducted into reducing the thickness of the depletion layer or reducing the size of the photoelectric conversion area to achieve an increase in the speed of photoelectric conversion.
As a device that uses the interaction of incident light and a surface plasmon, JP-A-2000-171763 (Patent Document 6) discloses an optical transmission device in which a metal film having an aperture and periodic surface variations is used to greatly enhance the intensity of light that is propagated through the aperture. This publication states that, even with a single aperture, the provision of rows of periodic grooves around the aperture enables greater enhancement of light that is propagated through the aperture than a case that lacks periodic rows of grooves. However, it is known that in surface plasmon resonance, the total energy of transmitted light is attenuated compared to the incident light energy. According to Tineke Thio, H. J. Lezec, T. W. Ebbesen, K. M. Pellerin, G. D. Lewen, A. Nahata, and R. A. Linke in “Giant optical transmission of sub-wavelength apertures: physics and applications,” Nanotechnology, Vol. 13, pp. 429-432 (Non-Patent Document 1), the total energy of light that is transmitted through an aperture having a diameter of 40% of the wavelength or less is attenuated to 1% or less of the incident light energy despite the use of surface plasmon resonance. As a result, a high SN ratio cannot be obtained in photoelectric conversion even when the optical transmission device disclosed in JP-A-2000-171763 is used to irradiate light that has been propagated from the aperture of the optical transmission device onto a photoreception device.
JP-A-2001-291265 (Patent Document 7) discloses a read/write head for an optical data recording medium that, by using near-field optics to improve the recording density of an optical data storage medium, both irradiates light onto an optical recording medium by way of an opening having diameter equal to or less than the wavelength and enhances the intensity of light transmitted through the aperture by means of the above-described surface plasmon resonance. In JP-A-2004-061880 (Patent Document 8), it is described that the read/write head disclosed in the above-described JP-A-2001-291265 does not use transmitted light that has passed through the aperture and then propagated to a remote location, but rather, uses a minute light spot that is formed in proximity to the aperture by near-field light (evanescent light). In the case of an optical data storage medium, the absorption coefficient of light in the storage medium can be raised to a high level, whereby all photons within a minute range such as a light spot produced by near-field light can be absorbed in the storage medium to enable the formation of minute recording pits. However, it is believed that when near-field light is introduced into a photodiode, the relatively low light absorption coefficient of the material that makes up the photodiode prevents irradiation of light to positions deep in the photodiode, whereby a sufficient photodetection current is not observed.
The reference documents cited in the present specification are listed below:
Patent Document 1: Japanese Patent Application Laid-open No. Sho-59-108376 (JP-A-59-108376);
Patent Document 2: Japanese Patent Application Laid-open No. Hei-8-204225 (JP-A-08-204225);
Patent Document 3: Japanese Patent Application Laid-open No. Hei-8-204226 (JP-A-08-204226);
Patent Document 4: Japanese Patent Application Laid-open No. Hei-10-509806 (JP-A-10-509806);
Patent Document 5: Japanese Patent Application Laid-open No. 2002-76410 (JP-A-2002-076410);
Patent Document 6: Japanese Patent Application Laid-open No. 2000-171763 (JP-A-2000-171763);
Patent Document 7: Japanese Patent Application Laid-open No. 2001-291265 (JP-A-2001-291265);
Patent Document 8: Japanese Patent Application Laid-open No. 2004-61880 (JP-A-2004-061880);
Non-Patent Document 1: Tineke Thio, H. J. Lezec, T. W. Ebbesen, K. M. Pellerin, G. D. Lewen, A. Nahata, and R. A. Linke, “Giant optical transmission of sub-wavelength apertures: physics and applications,” Nanotechnology, Vol. 13, pp. 429-432.